1. Field of the Invention
The present invention relates to a duobinary optical transmitter employing a duobinary optical transmission technique.
2. Description of the Related Art
In general, a dense wavelength division multiplexing (DWDM) optical transmission system has an excellent communication efficiency as it can transmit an optical signal having multiple channels with different wavelengths through a single optical fiber. Also, the DWDM system is capable of transmitting a larger amount of signals at a lower transmission speed by increasing the number of channels. Accordingly, the DWDM systems are now widely used in ultra-high speed internet traffic networks. Currently, systems capable of transmitting more than a hundred channels through a single optical fiber, using the DWDM technology, are commonly used. Various research is being actively conducted to develop a system which can transmit even higher than two hundred channels of 40 Gbps through a single optical fiber simultaneously at a transmission speed of greater than 10 Tbps.
However, the transmission capacity is restricted due to severe interference and distortion between channels if the channel spacing is less than 50 GHz using the conventional non-return-to-zero (NRZ) method to modulate the light intensity. Transmission distance is restricted in the high-speed transmission of greater than 10 Gbps since a direct current (DC) frequency component of a conventional binary NRZ transmission signal and a high frequency component spread during the modulation cause non-linearity and distribution when the binary NRZ transmission signal propagates in an optical fiber medium.
Recently, optical duobinary technology has been highlighted as an upcoming transmission technology capable of overcoming the above transmission restriction due to chromatic dispersion effect. A duobinary modulation method has a characteristic in that information is loaded based on the intensity of an optical signal and the phase of a signal is reversed at a ‘0’ bit. Since a duobinary signal has a narrower bandwidth than that of a conventional OOK signal, it is advantageous in reducing channel widths in a DWDM optical transmission system. Also, since the duobinary signal has a strong immunity against optical fiber chromatic dispersion, the duobinary signal can be transmitted two or three times further in comparison with a case of using an OOK signal. Furthermore, since the duobinary signal doesn't have a carrier tone component (i.e., DC frequency component) in an optical spectrum, the duobinary signal is advantageous in that it is strong against the stimulated Brillouin scattering (SBS).
FIG. 1 illustrates a conventional duobinary optical transmitter.
Referring to FIG. 1, the conventional duobinary optical transmitter includes: a pulse pattern generator (PPG) 10 for generating a 2-level electrical pulse signal; a precoder 20 for encoding the 2-level NRZ electrical signal; low pass filters 30 and 31 for changing the 2-level NRZ electrical signals outputted from the precoder 20 into 3-level electrical signals and reducing the bandwidth of the signals; modulator driving amplifiers 40 and 41 for amplifying the 3-level electrical signals to output optical modulator driving signals; a laser source 50 for outputting a carrier; and a Mach-Zehnder interferometer type optical intensity modulator 60.
The 2-level electrical pulse signals generated from the pulse pattern generator 10 are encoded in the precoder 20, and the 2-level binary signals outputted from the precoder 20 are inputted in the low pass filters 30 and 31, respectively. It is ideal that the low pass filters 30 and 31 are cosine2-shapedfilters. However, the low pass filters 30 and 31 may be approximately realized by using Bessel-Thomson filters. In a case in which a bandwidth of the low pass filters 30 and 31 have a bandwidth of −3 dB corresponding to ¼ of a binary data speed, for example, 2.5 GHz filters in the case of 10 Gb/s data, binary signals having undergone the low pass filters 30 and 31 are changed into band-limited ternary signals. That is, each of the low pass filters 30 and 31 has a bandwidth corresponding to about ¼ of the clock frequency of the 2-level binary signals. This excessive restriction to the bandwidth causes interference between codes, thereby changing the 2-level binary signal to a 3-level duobinary signal.
The 3-level duobinary signals are amplified in the modulator driving amplifiers 40 and 41 and then utilized as signals for driving the Mach-Zehnder interferometer type optical intensity modulator 60. The phase and light intensity of the carrier outputted from the laser source 50 are modulated by a driving signal of the Mach-Zehnder interferometer type optical intensity modulator 60, so that the modulator 60 outputs a 2-level optical duobinary signal modulated from the carrier. Herein, the positions of the low pass filters 30 and 31 and the modulator driving amplifiers 40 and 41 may be switched with each other.
Note that since a duobinary signal generated in the optical transmitter greatly depends on the characteristic of the low pass filter, the performance of the optical transmitter is greatly changed according to the pattern length of an applied binary signal. Also, using a Mach-Zehnder modulator, the optical transmitter is very sensitive to a change of a bias voltage. Therefore, when a bias voltage of the Mach-Zehnder modulator is changed due to the temperature change of the optical transmitter and so forth, the performance of a system can be deteriorated.
Such a duobinary optical transmitter has problems in that distribution is strong and a signal bandwidth is narrow. In order to solve these problems, a duobinary optical transmitter using a phase modulator and an optical filter has been proposed.
FIG. 2 is a block diagram illustrating a duobinary optical transmitter using a phase modulator and an optical filter. In FIG. 2, a precoder 110, a driving amplifier 120, and a laser source 130 are identical to those shown in FIG. 1, so the detailed description of those will be omitted.
Referring to FIG. 2, a binary data signal encoded by the precoder 110 is applied to an optical phase modulator 140 through the driving amplifier 120, and the optical phase modulator 140 modulates the phase of an inputted optical signal. A phase-modulated optical signal is converted into a duobinary signal through an optical filter 150—for example, 7 GHz filter in a case of 10 Gb/s data—which has a bandwidth corresponding to about 70% of a binary signal transmission speed.
With the optical transmitter shown in FIG. 2, although a generated duobinary signal has some lower immunity against the chromatic dispersion of an optical fiber in comparison with that of the optical transmitter shown in FIG. 1, the dependency problem according to a bias position of the Mach-Zehnder modulator and a pattern length appearing in the optical transmitter shown in FIG. 1 can be solved. However, the optical transmitter of FIG. 2 requires an optical filter having a narrow passband as well as an excellent dispersion characteristic, and thereby its realization is not easy.